Growth and Differentiation Factor-8 (GDF-8), also known as myostatin, is a member of the Transforming Growth Factor-beta (TGF-β) superfamily of structurally related growth factors, all of which possess important growth-regulatory and morphogenetic properties (Kingsley et al. (1994) Genes Dev. 8:133-46; Hoodless et al. (1998) Curr. Topics Microbiol. Immunol. 228:235-72). GDF-8 is a negative regulator of skeletal muscle mass, and there is considerable interest in identifying factors which regulate its biological activity. For example, GDF-8 is highly expressed in the developing and adult skeletal muscle. The GDF-8 null mutation in transgenic mice is characterized by a marked hypertrophy and hyperplasia of the skeletal muscle (McPherron et al. (1997) Nature 387:83-90). Similar increases in skeletal muscle mass are evident in naturally occurring mutations of GDF-8 in cattle (Ashmore et al. (1974) Growth 38:501-507; Swatland and Kieffer (1994) J. Anim. Sci. 38:752-757; McPherron and Lee (1997) Proc. Natl. Acad. Sci. U.S.A. 94:12457-12461; and Kambadur et al. (1997) Genome Res. 7:910-915). Recent studies have also shown that muscle wasting associated with HIV-infection in humans is accompanied by increases in GDF-8 protein expression (Gonzalez-Cadavid et al. (1998) PNAS 95:14938-43). In addition, GDF-8 can modulate the production of muscle-specific enzymes (e.g., creatine kinase) and modulate myoblast cell proliferation (WO 00/43781).
In addition to its growth-regulatory and morphogenetic properties in skeletal muscle, GDF-8 may also be involved in a number of other physiological processes (e.g., glucose homeostasis), as well as abnormal conditions, such as in the development of type 2 diabetes and adipose tissue disorders, such as obesity. For example, GDF-8 modulates preadipocyte differentiation to adipocytes (Kim et al. (2001) B. B. R. C. 281:902-906).
Like TGF-β-1, -2, and -3, the GDF-8 protein is synthesized as a precursor protein consisting of an amino-terminal propeptide and a carboxy-terminal mature domain (McPherron and Lee, 1997, supra) as well as a signal sequence which directs the protein to the extracellular domain and is also cleaved from the protein. It is believed that before cleavage of the propeptide, the precursor GDF-8 protein forms a homodimer. The amino-terminal propeptide is then cleaved from the mature domain and the cleaved propeptide may remain noncovalently bound to the mature domain dimer, inhibiting its biological activity (Miyazono et al. (1988) J. Biol. Chem. 263:6407-6415; Wakefield et al. (1988) J. Biol. Chem. 263:7646-7654; and Brown et al. (1990) Growth Factors 3:35-43). It is believed that two GDF-8 propeptides bind to the GDF-8 mature dimer (Thies et al. (2001) Growth Factors 18:251-259). Due to this inactivating property, the propeptide is known as the “latency-associated peptide” (LAP), and the complex of mature domain and propeptide is commonly referred to as the “small latent complex” (Gentry and Nash (1990) Biochemistry 29:6851-6857; Derynck et al. (1995) Nature 316:701-705; and Massague (1990) Ann. Rev. Cell Biol. 12:597-641). The mature domain is believed to be active as a homodimer when the propeptide is removed. Other proteins are also known to bind to GDF-8 or structurally related proteins and inhibit their biological activity. Such inhibitory proteins include follistatin (Gamer et al. (1999) Dev. Biol. 208:222-232) and follistatin-related proteins.
Further, a number of human and animal disorders are associated with loss of or functionally impaired muscle tissue, including muscular dystrophy, muscle atrophy, congestive obstructive pulmonary disease, muscle wasting syndrome, sarcopenia, and cachexia. To date, very few reliable or effective therapies exist for these disorders. The terrible symptoms associated with these disorders could be substantially reduced by employing therapies that increase the amount of muscle tissue in patients suffering from the disorders. Such therapies would significantly improve the quality of life for these patients and could ameliorate many effects of these diseases. Thus, there is a need in the art to identify new therapies that contribute to an overall increase in muscle tissue in patients suffering from these disorders.
The present invention fills this need by providing modified and stabilized GDF propeptides that retain their biological activity and inhibit the activity of GDF proteins. The modified propeptides of the invention may be used to treat human or animal disorders in which an increase in muscle tissue would be therapeutically beneficial.